Silting prevention controller

ABSTRACT

A relation between an instruction current value (is) to be given to a solenoid valve and its supply time range (t) is predetermined, and a silting prevention current instruction (is) is given to the solenoid valve according to the this relation. Then, silting can be prevented without fail even if a control current instruction is zero, and no fine vibration is caused in a hydraulic actuator when the control current instruction is being output.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a silting prevention control forpreventing silting from occurring in a space between a spool of asolenoid valve and a valve body.

2. Description of the Related Art

A pressure oil circulating through a hydraulic circuit often containspollutants such as debris. To remove such pollutants, the hydrauliccircuit is provided with a filter. Large pollutants can be removed bythe filter, but small pollutants can not be removed because they passthrough the filter. Therefore, the pollutants gradually accumulate inthe space between the valve body and the spool which is a moving memberof the solenoid valve. The pollutants collected in the space between thevalve body and the spool causes adverse effects such as the limitationof the movement of the spool. Such a phenomenon is called siltingcausing a problem.

It is assumed that a hydraulic actuator such as a hydraulic pump, ahydraulic motor or a hydraulic cylinder operates when the solenoid valveoperates.

Silting is caused when the spool in the valve body of the solenoid valvedoes not operate for a long period and an amount of the pressure oilflowing the space between the valve body and the spool is small.Conversely, pollutants hardly accumulate in the space between the valvebody and the spool when the pressure oil is flowing in an amountsufficient to operate the hydraulic actuator or the spool keepsoperating. Accordingly, silting does not occur.

Processing to superimpose a dither signal on a control currentinstruction which is applied to the solenoid valve in order to preventsilting is conventionally performed. For example, Japanese Utility ModelApplication Laid-Open Print No. 1-116812 indicates means for preventinga silting phenomenon by supplying an instruction electric current havinga dither signal of 100 to 400 Hz superimposed on a control currentinstruction to be applied to the solenoid valve.

Specifically, the dither signal of a constant amplitude is superimposedon the control current instruction and applied to the solenoid valve asshown in FIG. 5. By superimposition of the dither signal, the spool ofthe solenoid valve vibrates finely. Then, the flow of the pressure oilin the solenoid valve is changed. Accordingly, small pollutants flowwithout being accumulated in the space between the valve body and thespool, so that the silting is prevented from occurring.

But, the dither signal is generally not applied to the solenoid valvewhen the instruction current value is zero. It is because an outputvalue of the solenoid valve shall be zero but the dither signal might beoutput for its amplitude. When a state that the instruction currentvalue is 0 continues for a long time, a state that the dither signal isnot applied to the solenoid valve continues for a long time, and asilting prevention processing is not performed for a long time. Thehydraulic circuit often has a situation that the instruction currentvalue becomes zero. Therefore, there is a possibility that the siltingcannot be eliminated completely by the conventional processing ofsuperimposing a dither signal.

It is an object of the present invention to make it possible to preventsilting without fail even in a situation that a control currentinstruction is zero.

OBJECTS AND SUMMARY OF THE INVENTION

The present invention is a silting prevention control device comprisinga solenoid valve (13) in which a moving member slides with respect to avalve body according to a control current instruction to output apressure oil according to a moving position of the moving member and asubject equipment to be controlled which operates according to thepressure oil output from the solenoid valve (13), for preventing siltingfrom occurring in a space between the moving member and the valve body,wherein:

a relation between a time range and a current value of a currentinstruction to the solenoid valve (13) is predetermined within a rangethat the subject equipment to be controlled does not operate, and

a silting prevention current instruction is given to the solenoid valve(13) according to the predetermined relation.

The invention will be described specifically with reference to FIG. 1and FIG. 2.

FIG. 1 is a block diagram of a cooling fan drive using the presentinvention. In FIG. 1, servo piston 5 is a device to be controlled. Whenthe servo piston 5 operates, swash plate 2 a of a hydraulic pump 2 ismoved to change an amount of the pressure oil discharged from thehydraulic pump 2. The hydraulic pump 2 is driven by engine 1 todischarge the pressure oil. The pressure oil discharged from thehydraulic pump 2 is supplied to hydraulic motor 7. Upon the supply ofthe pressure oil to the hydraulic motor 7, cooling fan 3 mounted on anoutput shaft rotates. Radiator 9 is disposed to face the cooling fan 3,and when the cooling fan 3 rotates, heat of a coolant is radiated by theradiator 9. Hydraulic fan controller 20 outputs a current instructionaccording to an input engine speed, a coolant temperature and atemperature of the hydraulic oil of torque converter 11. The solenoidvalve 13 changes a valve position according to the current instructionoutput from the hydraulic fan controller 20. When the valve position ofthe solenoid valve 13 is changed, servo piston 5 operates.

FIG. 2 is a diagram showing a relation between instruction electriccurrent (is) for preventing silting, which is given to the solenoidvalve 13, and its supply time t. It is a relation determined in a rangethat the cooling fan 3 does not operate. According to the aboverelation, the silting prevention current instruction (is) is given tothe solenoid valve 13.

Therefore, the spool of the solenoid valve 13 moves to flow the pressureoil between the valve body and the spool in an amount not to operate thecooling fan 3. Accordingly, even if the control current instruction iszero, contaminants between the valve body and the spool can be removedwith reliability.

Because it is designed to apply the current instruction (is) capable ofpreventing the silting according to the predetermined relation, largecontaminants can surely be removed as compared with a conventional caseof superimposing a dither signal of a uniform size.

Besides, the silting prevention current instruction (is) is applied in arange that the cooling fan 3 does not operate, so that a dischargecapacity of the hydraulic pump 2 does not vary depending on a dithersignal having a predetermined frequency as it was caused heretofore.

The relation between the current instruction (is) and the supply time tis naturally different in its quantitative value depending on a size ofthe solenoid valve itself and a device to be controlled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of the invention;

FIG. 2 is a diagram showing a relation between an instruction currentvalue and a supply time range;

FIGS. 3(a), 3(b) and (3(c) are diagrams showing a relation between timeand a current instruction;

FIG. 4 is a diagram showing a procedure of silting preventionprocessing; and

FIG. 5 is a diagram showing a current instruction having a dither signalsuperimposed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of a silting prevention controller to which the presentinvention pertains will be described with reference to the accompanyingdrawings. It is assumed in the following description that the presentinvention is applied to a pump discharge capacity controller of acooling fan drive controller.

FIG. 1 shows a block diagram of the embodiment.

This pump discharge capacity controller adds control current instructioni output from the hydraulic fan controller 20 to electromagneticsolenoid 13 a of the solenoid valve 13 to change the valve position,thereby operating the servo piston 5.

The hydraulic fan controller 20 controls to drive the swash plate 2 a(capacity) of the hydraulic pump 2 by generating the control currentinstruction i according to an input signal and adding the controlcurrent instruction i to the electromagnetic solenoid 13 a of thesolenoid valve 13 to change the valve position of the solenoid valve 13.

The servo piston 5 is a capacity control member for moving the swashplate 2 a of the hydraulic pump 2 to change an angle of the swash plate.The servo piston 5 is moved to a position according to a pivot angle ofthe swash plate 2 a, namely displacing capacity Q of the hydraulic pump2.

The solenoid valve 13 is comprised of the valve body and the movingmember spool. When the control current instruction i is entered thesolenoid valve 13, the spool is moved to a position according to thesize of the instruction current value i and slides between the valvebody. According to the input control current instruction i, the solenoidvalve 13 moves between a valve position to supply the pressure oil(discharge pressure oil of the hydraulic pump 2) to a large diameterside of the servo piston 5 and a valve position to discharge thepressure oil from the large diameter side of the servo piston to tank 6.The solenoid valve 13 is changed its valve position as the controlcurrent instruction i output from the hydraulic fan controller 20 isadded to the electromagnetic solenoid 13 a and applies a pilot pressureproportional to the control current instruction i to an oil pressurechamber on the large diameter side of the servo piston 5.

The pressure to be output from the solenoid valve 13 to the largediameter side of the servo piston 5 increases as the control currentinstruction i applied to the solenoid valve 13 becomes large. And, thedisplacing capacity (discharge capacity) Q of the hydraulic pump 2decreases as the control current instruction i to be applied to thesolenoid valve 13 increases.

Thus, the hydraulic fan controller 20 outputs the control currentinstruction i corresponding to the displacing capacity Q of thehydraulic pump 2 to the solenoid valve 13 to control flow rate Q perrotation discharged from the hydraulic pump 2. Accordingly, the flowrate of the pressure oil being supplied to the hydraulic motor 7 iscontrolled, and the speed of the cooling fan 3 is controlled.

The variable displacement hydraulic pump 2 is a drive hydraulic sourcefor the cooling fan 3.

The hydraulic pump 2 is driven by the engine 1 as a drive source. Theengine 1 is provided with engine speed sensor 4 for detecting rotationsNe of the engine 1, namely input rotations Ne of the hydraulic pump 2.The speed sensor 4 can be for example a pulse pickup.

The hydraulic pump 2 is composed of, for example, a swash plate typepiston pump. The swash plate 2 a of the hydraulic pump 2 is moved, sothat the displacing capacity (discharge capacity) Q of the hydraulicpump 2 is changed.

The displacing capacity (discharge capacity) of the hydraulic pump 2 ischanged as the servo piston 5 operates.

The hydraulic pump 2 draws in the pressure oil from the tank 6 anddischarges it from a pressure oil discharge port.

The pressure oil discharged from the hydraulic pump 2 is supplied to apressure oil supply port of the fan drive hydraulic motor 7 through pipe8. The hydraulic motor 7 is a fixed capacity type hydraulic motor.

The output shaft of the hydraulic motor 7 is provided with the coolingfan 3.

The hydraulic motor 7 is driven to rotate as the pressure oil dischargedfrom the hydraulic pump 2 is supplied from the pressure oil supply portto rotate the cooling fan 3. The pressure oil discharged from thedischarge port of the hydraulic motor 7 is returned to the tank 6through pipe 8 a.

A coolant (cooling water) as a cooling medium for the engine 1 is guidedto the radiator 9. The heat of the coolant is radiated by the radiator9. The cooling fan 3 is disposed to face the radiator 9.

The coolant is cooled as the cooling fan 3 rotates. Temperature sensor10 for detecting temperature Tc of the coolant is mounted on theradiator 9.

The torque converter 11 is operated by the engine 1. Temperature sensor12 for detecting a temperature of hydraulic fluid for the torqueconverter 11, namely temperature Ttc of a torque converter (T/C) fluid.

The pressure oil to be discharged from the hydraulic pump 2 is supped toan unshown hydraulic cylinder. The hydraulic cylinder is operated by thepressure oil. The temperature sensor 12 may be used as a sensor fordetecting a temperature of the pressure oil of the hydraulic cylinder.The oil temperature of the hydraulic cylinder can be detected instead ofthe detection of the fluid temperature of the torque converter 11.Besides, a sensor for detecting the temperature of a pilot pressure oilpassing through the solenoid valve 13 can be replaced by the temperaturesensor 12.

Then, an operation of the hydraulic fan controller 20 will be described.

The hydraulic fan controller 20 outputs a current instruction to controla rotation speed of the cooling fan 3. The hydraulic fan controller 20is mainly comprised of pulse counter 21, ENG speed converter 22, targetfan speed calculation unit 23, A/D converter 24, control temperatureconverter 25, pump swash plate angle calculation section 26, solenoidvalve output converter 27 and silting prevention control unit 28.

The A/D converter 24 of the hydraulic fan controller 20 converts coolanttemperature detection signal Tc and torque converter fluid temperaturedetection signal Ttc from an analog signal to a digital signal andinputs them into the control temperature converter 25.

A pulse indicating engine speed detection signal Ne is counted by thepulse counter 21, converted to engine speed ENG RPM in an engineeringunit having a value corresponding to a size of the count value by theENG speed converter 22 and entered the target fan speed calculation unit23.

The control temperature converter 25 converts the coolant temperaturedetection signal Tc and the torque converter fluid temperature detectionsignal Ttc into the control temperature T.

The target fan speed calculation unit 23 determines target fan speed FANaccording to the input control temperature T and engine speed ENG RPM.

For example, when the control temperature T is less than 80° C., it isconsidered that the cooling (torque converter 11) of the engine 1 iscooled sufficiently, and target control value FAN RPM of the rotationspeed of the cooling fan 3 is not determined. At this time, theinstruction current value i is determined to make the swash plate 2 a ofthe hydraulic pump 2 have a minimum pivot angle.

On the other hand, when the control temperature is 80° C. or more,target fan speed FAN RPM according to the control temperature T isdetermined according to the predetermined relation.

When the target fan speed FAN is determined, a process of determining atarget swash plate pivot angle of the hydraulic pump 2, namely thetarget flow rate Q per rotation, is performed by the pump swash plateangle calculation unit 26.

As described above, the target flow rate Q of the hydraulic pump 2 isdetermined according to the target fan speed FAN, the fixed capacity Mof the hydraulic motor 7 and the rotation speed ENG RPM of the engine 1.

The solenoid valve output converter 27 determines the instructioncurrent value i corresponding to the target flow rate Q. The instructioncurrent value i is converted from the digital signal to the analogsignal and output as the control current instruction i to the solenoidvalve 13.

The silting prevention control unit 28 is predetermined to have theinstruction current value (is) for preventing silting to be added to thesolenoid valve 13, its supply time range t and the cycle T as shown inFIG. 2 and FIG. 3. And, according to the predetermined relation, thesilting prevention process shown in FIG. 4 is performed as describedlater in the specification to control so to output the siltingprevention current instruction (is) from the solenoid valve outputconverter 27.

The solenoid valve output converter 27 converts the silting preventioninstruction current value (is) from the digital signal to the analogsignal and outputs the silting prevention current instruction (is) tothe solenoid valve 13.

Then, a relation between the silting prevention instruction currentvalue (is) and its supply time range t predetermined by the siltingprevention control unit 28 will be described with reference to FIG. 2.

FIG. 2 shows the relation between the silting prevention instructioncurrent value (is) and its supply time range t supplied to the solenoidvalve 13. FIG. 2 also shows the presence or not of an effect of thecurrent instruction upon the operation of the cooling fan 3.

In FIG. 2, each point indicated by a circle () shows a relation notaffecting on the operation of the cooling fan 3 in the end.

Each point indicated by a triangle (▴) shows a relation of a limitbeginning to affect upon the operation of the cooling fan 3. At thepoint indicated by the triangle (▴), the rotation speed of the coolingfan 3 is variable but the sound generated by the cooling fan 3 is notfelt peculiar. In other words, line L2 by which the triangles (▴) areconnected indicates a limit line of beginning to affect on the operationof the cooling fan 3.

Each point indicated by a square (▪) shows a relation affecting upon theoperation of the cooling fan 3. At the point indicated by the square(▪), the rotation speed of the cooling fan 3 is variable and the soundgenerated by the cooling fan 3 is felt peculiar. Line L1 by which thesquares (▪) are connected indicates a line affecting upon the operationof the cooling fan 3.

For example, as indicated by A, giving the current instruction (is) withinstruction current value is 1 and supply time range t1 does not affectupon the operation of the cooling fan 3.

As indicated by B, giving the current instruction (is) with instructioncurrent value is 2 and supply time range t2 changes the rotation speedof the cooling fan 3 but the sound of the cooling fan 3 is not feltpeculiar.

But, as indicated by C, giving the current instruction (is) withinstruction current value is 3 and supply time range t3 to the solenoidvalve 13 changes the rotation speed of the cooling fan 3, and the soundof the cooling fan 3 changed is felt peculiar.

Therefore, in FIG. 2, points indicated by  and positioned below thelimit line L2 are determined to be the silting prevention currentinstruction (is) to be output to the solenoid valve 13.

This limit line L2 may be determined as a limit line where the operationof the servo piston 5 is began to be affected.

The current instruction (is) of the limit affecting on the operation ofthe cooling fan 3, namely the limit line L2 shown in FIG. 2, is variabledepending on a magnitude of the temperature of the pilot pressure oilpassing through the solenoid valve 13. Limit line L2′ shown in FIG. 2 isa limit line when the oil temperature is high, and limit line L2″ is alimit line when the oil temperature is low.

Therefore, it is desirable to determine that the silting preventioncurrent instruction (is) (mark ) to be output to the solenoid valve 13is not higher than the limit line L2′ when the oil temperature is highconsidering that the oil temperature may reach an upper limit and nothigher than ½ of the limit line L2′ considering variations in theaccuracy of equipment. For example, the silting prevention currentinstruction (is) to be output to the solenoid valve 13 shown in FIG. 2is determined to be the point indicated by A with the instructioncurrent value is 1 and the supply time range t1.

Where temperature detection means are disposed to control according to atemperature and the accuracy of equipment can be assured, a value littlesmaller than the limit line L2′ may be determined as the instructionvalue to maximize the silting effect.

Referring to FIG. 3, a relation between time τ and electric currentinstructions (i, is) will be described. In FIG. 3, the horizontal axisindicates the time τ, and the vertical axis indicates the controlcurrent instruction i and the silting prevention current instruction(is) to be given to the solenoid valve 13.

FIG. 3(a) shows that the control current instruction i is given to thesolenoid valve 13 but the instruction current value is less than thethreshold value is 1.

Instead of the control current instruction i with time τ1, the siltingprevention current instruction (is) with the instruction current valueis 1 and the supply time range t1 is given to the solenoid valve 13.

Then, instead of the control current instruction i, the same siltingprevention current instruction (is) is given to the solenoid valve 13 atrespective time τ2 . . . in each cycle T.

FIG. 3(b) shows that the control current instruction i is not given tothe solenoid valve 13, namely the current value of the control currentinstruction i is zero.

The silting prevention current instruction (is) with the instructioncurrent value is 1 and the supply time range t1 at time τ3 is given tothe solenoid valve 13.

Then, the same silting prevention current instruction (is) is given atrespective time τ4 . . . in each cycle T.

FIG. 3(c) shows that the silting prevention current instruction (is) isgiven only when the instruction current value of the control currentinstruction i becomes less than the threshold value is 1.

The silting prevention current instruction (is) is not given to thesolenoid valve 13 between time τ5 and time τ6 when the instructioncurrent value of the control current instruction i is not less than thethreshold value is 1.

But, the silting prevention current instruction (is) is given to thesolenoid valve 13 at time τ6 and later when the instruction currentvalue of the control current instruction i becomes less than thethreshold value is 1. In other words, the silting prevention currentinstruction (is) is given to the solenoid valve 13 at time τ7 after alapse of the cycle T after the instruction current value of the controlcurrent instruction i becomes less than the threshold value is 1.

A procedure of the silting prevention process which performed by thesilting prevention control unit 28 of FIG. 1 will be described withreference to the flow chart of FIG. 4. Description will be made alsoreferring to FIGS. 3(a) to 3(c).

As described above, the silting prevention control unit 28 is previouslydetermined to have the silting prevention current instruction (is) withthe instruction current value (is) and the supply time range t1according to the relation shown in FIG. 2. The cycle T to output thesilting prevention current instruction (is) to the solenoid valve 13 isalso determined in advance.

As shown in FIG. 3(a), FIG. 3(b) or shown as the time τ6 in FIG. 3(c),when the instruction current value of the control current instruction ibeing applied to the solenoid valve 13 becomes less than the thresholdvalue is 1 (Yes in step 401), it is judged that there is not anappropriate flow of the pressure oil in the space between the valve bodyand the spool of the solenoid valve 13 and the silting preventionprocess is necessary, and a counter for counting the cycle counts up thecount value Tcount from zero (step 402). When the instruction currentvalue i of the control current instruction i being applied to thesolenoid valve 13 becomes not less than the threshold value is 1 in step401 (No in step 401), it is judged that there is an appropriate flow ofthe pressure oil in the space between the valve body and the spool ofthe solenoid valve 13 and the silting prevention process is notnecessary, and the count value Tcount of the counter is reset to zero.For example, the count value Tcount is reset to zero when the time τreaches τ5 in FIG. 3(c) (step 403).

When the count value Tcount is the predetermined cycle T or below (No instep 404), the process goes to step 401, and the same process isperformed again.

When the count value Tcount becomes larger than the predetermined cycleT (Yes in step 404), the silting prevention process is performed.

Specifically, when the control current instruction i is given to thesolenoid valve 13 as shown in FIG. 3(a) and FIG. 3(c), the siltingprevention current instruction (is) of the instruction current value is1 is given instead of the control current instruction i to the solenoidvalve 13 at time τ1, τ2 of FIG. 3(a) or time τ7 of FIG. 3(c). And, whenthe control current instruction i is not given to the solenoid valve 13as shown in FIG. 3(b), the silting prevention current instruction (is)with the instruction current value is 1 is given to the solenoid valve13 at time τ3, τ4 of FIG. 3(b).

In other words, the solenoid valve output converter 27 outputs thesilting prevention current instruction (is) with the instruction currentvalue is 1 to the solenoid valve 13 (step 405). At the time, the countvalue Tcount is reset to zero (step 406).

When the silting prevention current instruction (is) started to beoutput, the counter for counting the current supply time range alsooperates to count up the count value tcount from zero. The counter forcounting the cycle counts up the count value Tcount from zero (step407).

The silting prevention current instruction (is) is continuously outputto the solenoid valve 13 until the count value tcount reaches thepredetermined supply time range t1 (step 408).

And, when the count value tcount becomes larger than the supply timerange t1 (Yes in step 408), the silting prevention process isterminated.

Specifically, when the control current instruction i is given to thesolenoid valve 13 as shown in FIG. 3(a) and FIG. 3(c), the controlcurrent i is given instead of the silting prevention current instruction(is) to the solenoid valve 13 after a lapse of the time t1 from the timeτ1, τ2 of FIG. 3(a) or the time τ7 of FIG. 3(c). As shown in FIG. 3 (b),when the control current instruction i is not given to the solenoidvalve 13, the instruction current value to the solenoid valve 13 isreturned to zero after a lapse of time t1 from the time τ3, τ4 of FIG.3(b). At this time, the count value tcount of the counter for countingthe current supply time range is reset to zero (step 409). After that,the process goes to step 401, and the same procedure is repeated.

According to the embodiment described above, the silting preventioncurrent instruction (is) is given to the solenoid valve 13 every cycle Twhen the control current instruction i is less than the threshold valueis 1, so that the spool of the solenoid valve 13 moves cyclically toflow the pressure oil in an amount not to operate the cooling fan 3.Therefore, even when the control current instruction i is zero,contaminants in the space between the valve body and the spool of thesolenoid valve 13 can be removed surely.

Larger contaminants can be removed securely as compared with theconventional case of superimposing the dither signal of a uniform sizebecause the current instruction (is) capable of preventing the siltingis applied according to the previously determined relation shown in FIG.2.

Besides, the cooling fan 3 is prevented from vibrating finely becausethe silting prevention current instruction (is) is applied in a rangenot to operate the cooling fan 3.

Thus, according to the above embodiment, silting can be preventedwithout fail even when the control current instruction i is zero. And,when the control current instruction i is output and the hydraulic motor7 and the cooling fan 3 are operating, the hydraulic motor 7 and thecooling fan 3 are prevented from having fine vibrations.

It is assumed in the above embodiment that the silting preventioncurrent instruction (is) is given to the solenoid valve 13 with theconstant cycle T. It is also possible to give the silting preventioncurrent instruction (is) with an indefinite cycle to the solenoid valve13 intermittently instead of the constant cycle T.

It may be designed to manually output the silting prevention controlinstruction. By manually operating the rotation speed limit switch 15, aspeed limit signal S is entered the hydraulic fan controller 20. Therotation speed limit signal S is an instruction signal for manuallyoutputting a silting prevention control instruction. The rotation speedlimit signal S is entered the silting prevention control unit 28.

In the embodiment shown in FIG. 4, the silting prevention currentinstruction (is) is given to the solenoid valve 13 even when the controlcurrent instruction i is output. But, the invention can also give thesilting prevention current instruction (is) to the solenoid valve 13only when the control current instruction i is zero as shown in FIG.3(b). Thus, power consumption can also be suppressed by giving thesilting prevention current instruction (is) to the solenoid valve 13only when the control current instruction i is zero.

An embodiment which changes the contents of the process to preventsilting according to a temperature of the pressure oil passing throughthe solenoid valve 13 will be described referring to FIG. 2.

First, an embodiment in which a temperature of the pressure oil passingthrough the solenoid valve 13 is detected and the relation shown in FIG.2 is modified according to the detected temperature of the pressure oilwill be described.

The pressure oil has a property that it is easy to flow at a hightemperature but not easy to flow at a low temperature. Therefore, thesilting prevention current instruction (is) necessary for preventing thesilting is variable depending on the oil temperature. Accordingly, therelation between the instruction current value (is) and the supply timerange t shown in FIG. 2 is compensated depending on the change in theoil temperature. Temperature Ta of the pressure oil passing through thesolenoid valve 13 is determined from the detection value of thetemperature sensor 12.

When the oil temperature Ta becomes high, the cooling fan 3 operateswith a small instruction current value and a small time range.Therefore, in order to prevent the cooling fan 3 from operating with asmall instruction current value and a small time range, the limit lineof FIG. 2 is changed from L2 to L2′ at a lower position. Thus, thesilting prevention current instruction (is) to be output to the solenoidvalve 13 is changed from the point A to the point A′, and the siltingprevention current instruction (is) with the instruction current valueis 0 and the supply time range t1 is given to the solenoid valve 13.

Conversely, when the oil temperature Ta becomes low, the cooling fan 3does not operate even if the instruction current value and the timerange are increased, so that the limit line of FIG. 2 is changed from L2to L2″. Thus, the silting prevention current instruction (is) to beoutput to the solenoid valve 13 is changed from the point A to the pointA″, and the silting prevention current instruction (is) with theinstruction current value is 2 and the supply time range t1 is given tothe solenoid valve 13.

As described above, even if the oil temperature is changed, the maximumsilting prevention effect can be obtained in a range that the coolingfan 3 does not operate.

In another embodiment, a temperature of the pressure oil passing throughthe solenoid valve 13 is detected, and the threshold value is 1 shown inFIG. 3 is changed according to the detected temperature of the pressureoil.

In other words, the control current instruction i necessary for theoperation of the cooling fan 3 by driving the hydraulic motor 7 ischanged according to the oil temperature Ta. And, the threshold value is1 shown in FIG. 3 is modified according to the change in the oiltemperature Ta.

When the oil temperature Ta becomes high, the cooling fan 3 is operatedby the control current instruction i with a small current value, and forexample the threshold value of FIG. 3(c) is changed to the thresholdvalue is 0 lower than is 1. When the instruction current value i of thecontrol current instruction i becomes less than the threshold value is 0lower than is 1, the silting prevention current instruction (is) isoutput to the solenoid valve 13.

Conversely, when the oil temperature Ta becomes low, the cooling fan 3operates according to the control current instruction i with a highcurrent value, and the threshold value of FIG. 3(c) is changed to thethreshold value is 2 higher than is 1. Thus, when the instructioncurrent value i of the control current instruction i becomes less thanthe threshold value is 2 higher than is 1, the silting preventioncurrent instruction (is) is output to the solenoid valve 13.

As described above, silting can be prevented in a range that the coolingfan 3 does not operate even if the oil temperature changes.

In the embodiment described above, a device having the cooling fan 3 asa finally controlled subject is assumed, and the silting preventionprocess is performed in a range that the cooling fan 3 does not operate.It is to be understood that the invention can also be applied to anysubjects to be controlled.

For example, the invention may be applied to a device having a hydraulicactuator such as a hydraulic cylinder, a hydraulic pump or a hydraulicmotor as a controlled subject to perform the silting prevention processin a range that the hydraulic actuator does not operate.

In the embodiments described above, the solenoid valve for controllingthe pilot pressure is assumed in order to prevent silting from occurringin the solenoid valve. But, the invention can be applied to any type ofsolenoid valves having any structures. For example, the invention canalso be applied to the prevention of silting from occurring in a flowrate direction control valve for controlling a flow rate and a directionof the pressure oil discharged from a main hydraulic pump.

What is claimed is:
 1. A silting prevention control device comprising asolenoid valve in which a moving member slides with respect to a valvebody according to a control current instruction to output pressurizedoil according to a moving position of the moving member, and a subjectequipment to be controlled which operates according to the pressurizedoil output from the solenoid valve, the silting prevention controldevice being adapted to prevent silting from occurring in a spacebetween the moving member and the valve body of the solenoid valve,wherein: a relation between a time range and a current value of asilting prevention current instruction to the solenoid valve ispredetermined within a range that the subject equipment to be controlleddoes not operate, and when a current value of the controlled currentinstruction is not more than a threshold value, the silting preventioncurrent instruction is given to the solenoid valve according to thepredetermined relation.
 2. A silting prevention control devicecomprising a solenoid valve in which a moving member slides with respectto a valve body according to current value and current applied timewidth of a control current instruction to output a pressurized oilaccording to a moving position of the moving member, and a subjectequipment to be controlled which operates according to the pressurizedoil output from the solenoid valve, for preventing silting fromoccurring in a space between the moving member and the valve body,wherein: a current value and a current applied time width are previouslyset as a silting prevention current instruction, the current value andthe current applied time width being such values that do not cause thesubject equipment to misfunction, and when the current value of thecontrol current instruction is not more than a threshold value, thesilting prevention current instruction is given to the solenoid.
 3. Asilting prevention control device comprising a solenoid valve and asubject equipment being controlled by the solenoid valve, the solenoidvalve having a moving member that slides within a valve body an amountrelative to a value of a control current instruction and an amountsufficient for preventing silting relative to a silting preventioncurrent instruction, the subject equipment operating in accordance withpressurized oil output from the solenoid valve according to a movingposition of the moving member sliding within the valve body in responseto the value of the control current instruction, and when the value ofthe control current instruction is maintained below a threshold valueand for a length of time sufficient for silting to occur within thesolenoid valve, the moving member is slid within the valve body inresponse to a value of the silting prevention current instruction forpreventing silting from occurring in a space between the moving memberand the valve body of the solenoid valve and an amount insufficient foroperating the subject equipment.